[Committed] Restore build on !TARGET_TRULY_NOOP_TRUNCATION targets

[Committed] Restore build on !TARGET_TRULY_NOOP_TRUNCATION targets

My sincere apologies to everyone, but especially Andrew Pinski
who warned me in advance that TRULY_NOOP_TRUNCATION results in
different code paths/optimizations on some targets. This restores
the build on nvptx-none (and presumably others) where mysteriously
(truncate:QI (reg:QI)) fails to be simplified to (reg:QI), which
is expected (everywhere) in my recently added self-tests.

2021-08-23  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* simplify-rtx.c (simplify_unary_operation_1): [TRUNCATE]:
	Handle case where the operand is already the desired mode.